Optical receiver with increased dynamic range

Abstract

An optical receiver with increased dynamic range includes a photodetector, a photodetector biasing network, an amplifier and a post-distortion network. The post-distortion network compensates for gain error in the amplifier, such that a composite output voltage is relatively linear with respect to input current. The dynamic gain responses of the amplifier and the post-distortion network are equal in magnitude and opposite in phase. Additionally, a signal from at least one internal node of the amplifier may be connected to the post-distortion network, in order to further improve performance.

Description

BACKGROUND OF THE INVENTION[0001]This application claims the benefit of U.S. Provisional Application No. 60 / 601,018, filed Aug. 12, 2004, entitled OPTICAL RECEIVERS AND AMPLIFIERS FOR LINEAR BROADBAND DISTRIBUTION SYSTEMS, the disclosure of which is herein incorporated by reference.FIELD OF THE INVENTION[0002]The present invention relates generally to circuits for optical receivers, and more particularly to a design for an optical receiver having increased dynamic range.DESCRIPTION OF THE RELATED ART[0003]The delivery of video services over communication systems such as Hybrid-Fiber-Coax (HFC), Fiber-To-The-Curb (FTTC), and Fiber-To-The-Home (FTTH) often necessitates the use of high dynamic range technologies to support legacy analog NTSC signal formats. These video systems all use amplitude modulated (AM) optical carriers and require an optical transmitter to modulate the information onto the light. They also require an optical receiver to demodulate and amplify the signal for use ...

AI Technical Summary

Benefits of technology

[0017]According to one embodiment of the present invention, an optical receiver with increased dynamic range includes a photodetector, a photodetector biasing network, an amplifier and a post-distortion network. The post-distortion network compensates for gain error in the amplifier, such that a composite output voltage is relatively linear with respect to input current. As measured at the output of the receiver, the dynamic gain responses of the amplifier and the post-distortion network are designed to be equal in magnitude and opposite in phase. Additionally, a signal from at least one internal node of the amplifier may be connected to the post-distortion network, in order to further improve performance.

[0019]According to a preferred method of the present invention, the dynamic range of an optical receiver may be increased by detecting an optical signal with a photodetector, the photodetector biased with a biasing network, applying an output current from the photodetector to an amplifier, and applying an output signal from the amplifier to a post-distortion network, wherein the post-distortion network compensates for gain error in the amplifier, such that a composite output voltage is relatively linear with respect to input current. The method may further comprise applying a signal from at least one internal node of the amplifier to the post-distortion network, in order to further improve performance.

Problems solved by technology

In the case of analog RF video signals, either excessive distortion or excessive noise will degrade customers' viewing experience.

Because of the spatial diversity of customers and the variable nature of optical link budgets in typical deployments, optical path losses can widely vary.

Unfortunately, optical receivers for the 1550 nm wavelength video portions of the FTTP system only support about 7 or 8 dB of dynamic range.

The small optical dynamic range of video optical receivers can make FTTP deployments more difficult since more effort must be expended to meet the relatively narrow optical input window.

Unfortunately larger transistor active area leads to increased power consumption and cost.

That is, a design specifically optimized for good distortion performance will have degraded noise performance, compared with a design which targets low noise.

It is also worth mentioning that poor distortion and noise performance affect systems differently depending on the type of content transmitted.

Raising Rfb leads directly to an increase in distortion.

It is not a good approximation for amplifiers fabricated from bipolar junction devices (BJT) due to the comparatively high base current and correspondingly high shot noise.

However, as previously stated, a larger Rfb implies that a larger output voltage Vout must be supported with good distortion characteristics by our Amplifier A. When Vout increases, so does the distortion generated in Amplifier A. This leads to a direct trade-off between noise and distortion performance in the circuit of FIG. 2.

First, because thermal noise contributions of each feedback resistor Rfb and Amplifier are independent from one another, noise power from these sources will be additive at the output.

Any imbalance of current flow into the separate amplifiers, or imbalance in the power series characteristics of the amplifiers, or imbalance in the characteristics of the push-pull combiner, will lead to a direct loss of 2nd order cancellation.

Should this imbalance become too large, the noise reduction properties will also degrade.

Winding of baluns and transformers is labor intensive and therefore expensive.

However, 2nd order distortions emanating in the differential amplifier output stage will not cancel without an output transformer or balun device, as in U.S. Pat. No. 5,239,402.

In summary, balancing noise, distortion, and cost are the primary challenges in the design of optical receivers.

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